Avant Propose

"Spicing
the Indian Ocean:The
Indonesian Throughflow
is the only low-latitude inter-ocean current between world oceans,
linking the Pacific to the Indian Ocean through boundary currents
along the western shores of the Pacific. The Indonesian Throughflow
sources the Indian Ocean with warm, less salty water from the North
Pacific Ocean via the Mindanao Current and water from the South
Pacific Ocean via the New Guinea Coastal Current. These Pacific
western boundary currents are important players in the global climate
system since theclosing
of the Panama Isthmus
during the Pliocene and thus cutting the exchange between tropical
Pacific and tropical Atlantic."

"Powering
winter storms:
In winter, the warm subtropical currents in the along the western
boundary of Pacific Basin supply enormous amounts of heat and
moisture into the atmosphere, feeding the North
Pacific storm tracks. As
cold and dry sub-polar air comes into contact with the warm water
carried poleward by the subtropical western boundary currents, heat
and moisture to fuel the storms are extracted from the surface
water."

"Rocking
the globe:
The Pacific is the home of the El
Nino, an inter-annual
ocean-atmosphere oscillation, which severely disrupts global weather
patterns worldwide. El Nino affects ecosystems, agriculture, tropical
cyclones, drought, bushfires, floods and other weather events
worldwide. This El Nino/Southern Oscillation (ENSO) is the most
prominent source of global climate variability. The low-latitude
western boundary currents in the Pacific are transporting mass into
the equatorial Pacific. The western boundary currents critically
influence the Western
Pacific Warm Pool (a region
of sea surface temperatures warmer than 28.5 C). In consequence,
they affect the life cycle of the El Nino/Southern Oscillation, the
East Asian monsoon and the Indian/Southeast Asian monsoon."

Western Boundary Currents - what?

Three
events, same special agent of change:
Western Boundary Currents.
These currents are a common marine agent with manifold impacts on the
global ocean circulation and climate. Thewarming
of the western coast of all continents
is the best-known feature that these currents trigger. However,
there are many more features as the examples of Pacific Ocean show.
Anyhow, how are these boundary
currents set-up? That
is the story to tell here, because the impacts of the western
boundary currents on the global ocean circulation and climate
variability are manifold.

Take three: winds, continents and rotation

The
winds stir the surface layer of the sea, driving waves, currents and
mixing. Water flows at the surface of the ocean mainly are to the
West in tropical zones and the East in temperate regions. Closer to
polar regions the circulation pattern gets a bit different. In the
south, the Circumpolar Current circles eastward the Antarctic
continent. In the north, the continents hinder such a mighty flow of
water around the globe.

Surface currents in the North Atlantic (model) -Credit:

Erik Behrens (distributed via imaggeo.egu.eu)

The
continents shaping the ocean basins of Atlantic Ocean, Indian Ocean,
and Pacific Ocean hinder ocean currents flowing around the globe,
with the Antarctic Circumpolar Current in the Southern Polar Ocean
being the sole exception. Otherwise, ocean currents arrange as big
gyres. The gyres in the tropical-temperate zones of the oceans show
gentle, broad flows towards the equator in the east and swift, narrow
and poleward currents in the west. The currents in the west, the Gulf
Stream, for example, flow quite close to the continent before they
get unstable and turn eastward into the open ocean.

The
rapid spinning of planet Earth causes the different water flows in
eastern and western parts of the oceans. The easterly winds in the
tropical regions, the westerly winds in the temperate regions, and
the confinement of the ocean between north-south boundaries together
alone would not cause this east-west asymmetry. The interplay of the
Earth's rotation, the winds, and the continents causes the narrow
western boundary currents and the broad eastern flow of the ocean
surface waters in the subtropical gyre.

The
earth rotation affects in a different manner a blob of water at the
pole or the equator. A blob of water exactly at the pole rotates
around its vertical axis. A blob of water at the equator is swung
around Earth without being rotated around its vertical axis. Between
both places, the situation changes smoothly from one extreme to the
other. Thus due to the rotation of the earth, blobs of water
experience a torque that depends on their latitude.

Blobs
of water situated at the same latitude have the same amount of torque
that stems from the rotation of the Earth but the contribution to
their torque that is arising from other sources may vary. Blobs of
surface water acquire additional torque from the winds blowing over
the sea when these winds vary in strength or direction over the blob.

If
a blob of water driven by the wind moves straight east-west, thus
along a latitude, then little happens to it beyond the contribution
to its torque that stems from the wind. However, if the same blob of
water moves poleward (or towards the equator), then the internal
repartition of its torque has to adjust. Adjustment is done by
changing the shape of the blog. Changing the blob's form requires
that water flows within the blob change. Finally, these changes
result in the east-west asymmetry of ocean currents, with narrow
western boundary currents and the broad eastern return-flows. The
western boundary currents have a second dynamic function. The torque
added by the wind has to be dissipated somewhere in the ocean.
Dissipation happens through the high lateral shear in the western
boundary currents and bottom friction.

The German research vessel METEOR off the coast of Brazil Credit: MARUM - Center for Marine Environmental Sciences (distributed via imaggeo.egu.eu)

[adapted
from [1]]: “Much
of modern wind-driven ocean circulation theory was derived from a
quest to understand these ocean currents. Knowledge of the effect of
the Earth’s rotation on western boundary currents, and of the wind
driven water flow [Ekman
transport],
led to ground-breaking advances: that wind stress is a driving agent
of ocean currents, but it is the horizontal change rather than the
absolute strength that is important; that latitudinal gradients in
the effect of Earth’s rotation on the horizontal motion cause a
flow intensification towards the west of the ocean basins; and that
ocean wind-forced ocean currents include the equatorial current
system, the low-latitude and subtropical western boundary currents.”

Name them, the Western Boundary Currents, some

Within the North Atlantic Ocean, theGulf Stream along the East Coast of the USA is a well-known western boundary current. The Gulf Stream is part of the subtropical gyre of the North Atlantic Ocean. Its counterpart in the subtropical gyre of the South Atlantic Ocean is theBrazil Current. Between both gyres flow the equatorial currents in the tropical belt. These currents link the two oceanic gyres along the continental borders. In the Atlantic Ocean, that link transports surface water northward along the American continent. Further in the south the subtropical gyre of the South Atlantic Ocean ties with theAntarcticCircumpolar Current and through theAgulhas Current with the subtropical gyre of the Indian Ocean.

Within the subtropical gyres of the Pacific Ocean, the western boundary currents are theKuroshio Current in the Northern Pacific Ocean, and theEast Australian Current in the Southern Pacific Ocean. The Pacific tropical belt western boundary currents include theMindanao Current northern of the Equator and the New Guinea Coastal Current south of the Equator. The circulation system of the equatorial Pacific connects the currents in both hemispheres.

Within the Indian Ocean, theSomalia Current along the east-coats of Africa is a western boundary current. That current changes the direction of its flow with the seasonal switch of the Monsoon winds. The surface flow of New Guinea Coastal Current in the Pacific Ocean exhibits the same annual feature. The New Guinea Coastal Current changes its direction with the monsoon season. The related under current, the New Guinea Coastal Undercurrent that is flowing in the depth of the sea, flows poleward all the year.

About Me

My professional education is physical oceanographer and physical limnology (PhD). I worked on marine and freshwater environments. Since two decades I work in the management of the European research programmes.
And for the lawyers, this blog represents my views only and not those of my employer.